The utility of many structural materials has been limited by, among other things, lack of strength, cracking and thermal expansion problems. Familiar examples are the fields of ceramic and clay materials, which have not generally been used for large scale structures due to lack of resistance to fracture under thermal stresses.
The failure mode of ceramics is often described as micro-structural deformations or micro-cracks partially caused by a non-homogeneous distribution of the particles and binders contained in the ceramic materials. It has been found that cracks propagate along a path in the two dimensional plane until the material fails.
The prior art has sought to ameliorate certain of these structural drawbacks by incorporating reinforcements selectively dispersed in a matrix. Platelets, fibers, whiskers, and minerals such as perlite and calcined grogs, are some of many such reinforcements that have been employed in ceramics materials. However, prior art reinforcements have been ill-suited for many large scale or otherwise demanding applications partly due to only marginal increase in fracture toughness. In particular, the tendency of many ceramic materials to exhibit catastrophic fatigue failure has severely limited the utility of ceramics in most structural and refractory applications. Also, the properties of ceramic materials have been difficult to predict due to imperfections created during green formation and sintering of the ceramics.
Currently known aggregates employed in high refractory bodies are spherical, tabular, or of random geometry. The primary engineering consideration is the packing efficiency. In general, a graded approach is practiced, wherein closed fractions of up to 5 different sizes are utilized to achieve near theoretical density. For example, Alcoa Corporation recommends the use of four different closed fractions of graded sizes in one of its commercial calcium aluminate cement products.
It is well known that smaller particles introduced and distributed in the interstices of packed larger particles reduce the porosity and pore size. The reverse is also true, where large particles added to finer particles displace fines and pores and reduce porosity. This model is called the Furnas model (C. C. Furnas, U.S. Bur. Mines Rep. Invst. 2894 (1928)). When the large particles in a packing are in contact, the theoretical maximum packing fraction PF.sub.max for a mixture of coarse, medium, and fine particles may be described as: EQU PF.sub.max =PF.sub.c +(1-PF.sub.c)PF.sub.m +(1-PF.sub.c) (1-PF.sub.m)PF.sub.f,
where PF.sub.c, PF.sub.m and PF.sub.f are the packing factors of the coarse, medium, and fine particles, respectively.
For example, McGeary experimentally achieved a packing density of 95% for a quaternary system of vibrated steel spheres of diameters 1.28, 0.155, 0.028, and 0.004 cm. (R. K. McGeary, J. Am. Ceram. Soc. 44 (10), 513-522 (1961)).
Dinger and Funk, however, realized that for densest packing, continuous particle size distribution IS preferred. They derived the following relationship: ##EQU1## where: F.sub.m (a)=the cumulative fraction finer than particles size a;
n=the slope in cumulative particle size versus particle size distribution curve; PA1 a.sub.max =the maximum particle size; and PA1 a.sub.min is the minimum particle size.
The packing density may increase as 1/n and as the range of sizes increase, and the packing density may exceed 80%. It should be noted that the above considerations apply to particles which are spherical in shape and under ideal packing conditions. As well, it should be noted that under idealized spherical particle packing conditions, such as that achieved by McGeary (95%), the remaining 5% (the matrix space) is characterized by a pattern of discrete voids of fragile geometry connected by very thin passages between the coordination points of the spheres in this system.
For example, the prior art has used point-to-point contact of particles of spherical and near spherical shapes to achieve better densities. Additionally, one can also establish line contact for rod-shaped particles to improve packing. Packing is enhanced further by using platelets, because of the established surface-to-surface contact. However, in reality, when rod-like and platelet-like particles are used, bridging occurs, contributing to hindered packing. Even under ideal conditions where packing hindrance is not considered, a consequence of using rod-like and platelet-like particles is the loss of isometric properties. Loss of isometric properties can render the structure susceptible to failure arising from in-plane shear and tensile forces.
The isometric properties of a material using rod or platelet-shaped particles are usually lost because the particles have a tendency to orient themselves parallel to their long dimensions, thereby giving rise to layering and the consequent loss of these properties.
Although surface-to-surface contact yields the highest packing densities, it must be accomplished uniformly in all dimensions to retain both maximum packing density and isotropy. Uniform surface-to-surface contacts surrounding spherical particles is very difficult to achieve. Uniform line-to-line and surface-to-surface contact of rods and platelets is virtually impossible to achieve.
One reason for the cracking problems of many structural materials is that incomplete homogenization of the media being cast persists throughout the fabrication process. Subsequently, when materials bearing these inhomogeneities (material gradients) are placed into service, applied and/or inherent (fabrication related), stresses localize at the sites of the inhomogeneities to cause stress gradients. When the total crack surface area energy requirement for that locale has reached its critical threshold, crack propagation ensues.
As reported by Lawrence Nielsen in Mechanical Properties of Polymers and Composites, Marcel Dekker (1974), composite materials may be defined as materials made up of two or more components and consisting of two or more phases. Such materials must be divided into three general classes: 1. Particulate-filled materials consisting of a continuous matrix phase and a discontinuous filler phase made up of discrete particles; 2. Fiber filler composites; and 3. Skeletal or interpenetrating network composites consisting of two continuous phases. Examples of this last class of materials include filled open-cell foams and sintered mats or meshes filled with some material.
Many commercial polymeric materials are composites, although they are not often considered as such. Examples include polyblends and ABS materials, foams, filled polyvinyl chloride formulations used in such applications as floor tile and wire coatings, filled rubbers, thermosetting resins containing a great variety of fillers, and glass fiber-filled plastics. There are many reasons for using composite materials rather than the simpler homogeneous polymers. Some of these reasons are: 1. Increased stiffness, strength and dimensional stability; 2. Increased toughness or impact strength; 3. Increased heat distortion temperature; 4. Increased mechanical damping; 5. Reduced permeability to gases and liquids; 6. Modified electrical properties; and 7. Reduced cost.
Not all of the above desirable features are found in any single composite. The advantages that composite materials have to offer must be balanced against their undesirable properties, which include complex rheological behavior and difficult fabrication techniques, as well as a reduction in some physical and mechanical properties.
The properties of composite materials are determined by the properties of the components, by the shape of the filler phase, by the morphology of the system, and by the nature of the interface between the phases. Thus, a great variety of properties can be obtained with composites just by alteration of the morphological or interface properties. An important property of the interface which can greatly affect mechanical behavior is the strength of the adhesive bond between the phases.
Nevertheless, the fabrication of large structures currently remains out of the realm of possibility due to limitations in conventional ceramic processing technology in shape forming and consolidation of large bodies. The consolidation and sintering require uniform temperatures throughout the firing step which is exceedingly difficult and this leads to density variations due to temperature gradients. Microstructural nonuniformity thus results leading to thermal shock damage as the vessel is placed in use. As will be seen, we believe that our invention provides an improvement in the area of composite formulation and manufacture.
To advance the understanding of this invention, it is our belief that an abstract of certain novelties involved as applied to a specific and challenging application would be of help.
With the above definition in mind, consider a one-piece vessel for the long term (100,000+ years) storage of radioactive or otherwise hazardous waste.
Practically speaking, the vessel would have to be large and capable of holding one or more kiloliters of material. The state of the art in this field comprises short term solutions, at best, involving stainless steel, reinforced concrete, or polymeric composite vessels.
The goal is a vessel large enough to be practical, and engineered in such a way as to ensure its long term integrity. Specific material selection would follow exhaustive analysis. It is our belief that alumina, being plentiful, relatively inexpensive, non-absorptive, and highly resistant to damage from radiation is a likely candidate for such analysis.
It is believed that the vessel could be manufactured in the following fashion:
1. Finite element analysis would be employed to determine the exact size and dimensions of the vessel.
2. Once decisions concerning geometry and dimensions have been made, body-specific concerns, such as aggregate dimensions, matrix/aggregate and interphase constituent chemistry, and degree of reticulation, might be addressed. Here it is believed that, where a vessel wall thickness dimension of 10-20 cm is considered, an aggregate of approximately 0.10 to 1.00 cm (as measured by its longest dimension), would be appropriate. When the reticulation property of the manufactured aggregates is utilized in a composite, the interstice-space between the aggregates will form the matrix. The material contained in this space is herein referred to as `matrix level reinforcement`. It should be realized that at high levels of aggregate reinforcement, i.e., greater than 75%, this matrix phase itself will be distinctly reticulate.
Matrix material candidates must exhibit high modulus of rupture, low thermal expansion coefficients and substantial resistance to thermal shock. Some candidates in this area of matrix material are alumina, mullite, the borosilicates, and lithium alumina silicate.
3. The aggregate can then be manufactured according to methods disclosed herein. Here it is believed that the principal reinforcement phase, when reticulated to achieve a 50 to 90 percent volume fraction (more or less), would leave a matrix phase of approximately 0.001 to 0.005 cm in thickness dimension. It is believed that this matrix phase (in the interest of achieving superior fracture resistance in the vessel) can itself be composed of a matrix and a reinforcement phase. Matrix level reinforcements would then be approximately 10 to 50 .mu.m. Once nanophase production technology for these aggregates has been developed, this graded fraction approach to matrix reinforcement may be carried to at least another level.
4. Casting of the green structure could be carried out after methods already known to those skilled in the art. Considering the vessel's large size and demanding end use, however, alternative approaches to the known methodology might be considered. Uniformity in wall thickness and reticulation of reinforcement phase(s) is considered critically important. One method for achieving this uniformity might involve a bi-axial centrifugal casting approach wherein the mold, after having been charged with a controlled amount of matrix/aggregate, would then be rotated about two perpendicularly opposed axes. Mechanical, acoustic and/or ultrasonic energy would then be employed to facilitate an even wall thickness and reticulation of the reinforcement phase(s).
5. Waste loading, inspection, and/or monitoring ports (if not provided for in the mold design), may be accommodated at this point. Waiting until the casting has gone through a debindering and/or calcining phase before undertaking this effort may be advisable, however. Indeed, it may prove advisable to wait until after the vessel has been fully densified before machining the necessary fittings.
6. It is believed that full densification (sintering) of the structure might follow conventional furnacing methodology. However, given the vessel's extraordinary size, maintenance of uniform heat dispersion with conventional methodology might be difficult, if not impossible. An alternative to the known art might follow a novel directed laser, plasma, microwave sintering approach.
Pure alumina does not respond well to microwave radiation. Dopants, in the form of parts-per-million concentrations of certain metallic elements, have been used to accomplish a coupling effect. It is believed that where borosilicate and lithium alumina silicate matrix constituents are considered, this coupling might be achieved. It must be remembered that the reinforcement phase(s) (as disclosed herein) have already been sintered as part of the aggregate production process, and need only be interphased with (sintered to) the matrix constituents to achieve full densification of the vessel.
Therefore, a task of this magnitude requires a reinforcement which improves, among other things, the fracture resistance of structural materials, such as ceramic materials, and methods for making the same. As well, the unique shapes of these system components (in the present case reinforcements), will require advanced methods of consolidation of these components into composite systems.
One aspect of the present invention are aggregates having unique three-dimensional shapes which are theoretically capable of packing to near 100% density without any void volume and using only one size fraction. This high level of packing efficiency is the result of surface-to-surface contact of the novel shapes in a three-dimensionally reticulate matrix. The reinforcements disclosed herein exhibit uniqueness of shape which lead to the ability to reticulate in all dimensions; therefore, previous considerations of packing spherical particles in various configurations are not generally applicable. The surface-to-surface contact or plane-based coordination obtainable with these novel forms will result in an interactive and mutually supportive relationship between the discrete components thereby improving upon the point-to-point and line-to-line based coordination of conventional reinforcement geometries.
In accordance with another aspect of this invention, individual aggregates of predetermined composition, size, density, and/or porosity may be employed in fluid bed systems where architectural soundness of component geometry and exposed surface area of component geometry are prime considerations. Example applications in this field include, but are not limited to, fluid bed components in coal combustion systems as well as components in engineering systems that use catalysts.
In accordance with another aspect of the present invention, a structural matrix is formed using aggregates of a consistent, predetermined, reticulate form. One example of such an aggregate is designated a Tetrajack, which may be described as a base tetrahedron (the base) with coincident tetrahedra joining its four faces. Another example of such an aggregate is the Tetratwin. The Tetratwin may be described as a class of shapes based upon a pair of twin tetrahedra joined coincidentally on a single face. Still another reticulate form the Starjack, has the shape of six obelisks extending from the six faces of a base cube.
In accordance with another aspect of the present invention, a reticulate fatigue resistant matrix material is formed exhibiting only minimal crack propagation thereby avoiding catastrophic brittle failure.
In accordance with yet another aspect of this invention, a reticulate matrix is formed using precursor aggregate of the claimed shapes. The precursor aggregate are then ignited (ashed), dissolved or otherwise removed, leaving voids now defined by the geometry of the reticulated precursors. Such a body would find application (for example), in the refractory field as thermal insulation where dimensional stability, controlled porosity, and resistance to thermal shock are prerequisites. Such reticulated matrix structures with controlled void size, shape, and distribution can also be used as filters and chemical reaction beds.
In accordance with another aspect of this invention, precursor aggregate may be reticulated to a predetermined dispersion, then replaced or transformed in a variety of ways (chemical vapor deposition, etc.) to yield a composite body heretofore unobtainable.
In accordance with yet another aspect of this invention, extruded forms reinforced with aggregates of geometries herein disclosed may be formed. It is believed that an aggregate of three-dimensionally reticulate geometry (herein disclosed as the Tetratwin), which has a long axis approximately twice the length of its width, may be of particular advantage here. In fact, the nature of the extrusion process lends itself admirably to the reticulation or alignment of the reinforcements with or without the application of additional energy at or near the point of extrusion.
In accordance with still another aspect of the invention, a matrix employing reticulated aggregate forms is cast and sintered to serve as a containment vessel, such as may be suitable for the long term containment of toxic or nuclear waste.
In accordance with another aspect of the invention, the above containment vessel may be constructed in the form of a reticulate aggregate and disposed in a containment storage field in a reticulate fashion, thereby optimizing the storage capacity of the overall field and protecting it from catastrophic, seismic activity-related damage. The engineered degree of reticulation (dispersion) would follow substantial site review and finite element analysis of the proposed containment geometry. The distance of dispersion (degree of reticulation/matrix space) would then be filled with a filler, such as ordinary sand, to cushion and further stabilize the reticulated containment forms.
The resulting vessel (example compositions might be alumina/alumina, mullite/alumina, borosilicate glass/alumina) is also highly resistant to thermal shock i.e., the structure's thermal coefficient of expansion does not jeopardize the vessel's integrity, even over wide temperature ranges. High thermal shock resistance is desirable when the vessel is subjected to rapid changes in temperature such as hot or cold radioactive waste materials. Also, the vessel's integrity is not jeopardized by uneven heat loading, such as might be experienced if "hot" elements were to precipitate and distribute themselves unevenly in the vessel.
In accordance with yet another aspect of the invention, a mold surface permitting the aggregate to be manufactured in sheet-like or "fabric" form wherein individual aggregate are joined each to the other by consistent "flash" elements is disclosed.
In yet another aspect of this invention, additional vibrational energy (from mechanical, ultrasonic, microwave or other sources in coherent or broadfield forms) is applied through the mold surfaces to the material being cast or press formed. This amalgam of potential vibrational candidates may be used singly or in combinations thereof. A continuous production process wherein green aggregates are press formed and then passed directly into a microwave sintering environment is envisioned.
In yet another aspect of this invention, a body may be composed of aggregate disposed in a reticulate matrix where the matrix itself is composed of aggregate of reticulate geometry. It is believed theoretically possible to employ reticulate aggregate geometry in a succession of aggregate to matrix closed fractions from less than several nanometers to more than several inches.
In yet another aspect of this invention, aggregates may be formed from mold surfaces employing a variety of conventional fabrication systems such as: hot isostatic pressing, cold isostatic pressing, injection molding, slip casting, drain casting, centrifugal casting, and gel casting. It is believed that a body resulting from the above approach to composition will represent a step forward in material strength and fracture toughness.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.